The financial support system for new built cruise ships I would imagine works like this:
From the cruise line:
The cruise line uses equity from older ships, cash on hand, and new loans from investors to finance the new project. Over time of profitability, these loans are paid off.
From the customer:
The customer pays a higher price initially until all the loans taken to finance the project are paid off. After the ship is adequately profitable, the price of cruises will begin to drop. This takes time.
As for physical limitations on home ports, most of the Florida ports (exempting Tampa), do not have any height limitations, and depth limitations are easily fixed by dredging, but I wonder the likelihood of these cities to want to spend large amounts of money on dredging. New York City (Cape Liberty, Manhattan, and Brooklyn) is limited by the height of the Verazanno Bridge, and as Tim mentioned, they [the cruise lines] are already heavily pushing the limits.
To me, the ideal size for a ship is anywhere from 70,000 - 140,000 tons, though I favor the 100,000 GRT ships.